The present invention relates to microcapsules having walls comprising polyurea obtained by reaction of polyisocyanates, guanidine compounds and optionally amines, to processes for producing these microcapsules, and to the carbonless copy papers produced from the microcapsules.
Carbonless copy papers are coated on their back with a layer of microcapsules. The microcapsules contain a solution in oil of a color former, so called because it is capable, in the presence of acidic developers, of effectuating a change in color from colorless to colored within a few seconds.
When a capsule-coated paper, then, is placed on a developer-primed paper in such a way that the capsules are in direct contact with the developer, a colored image will appear on the developer side on writing on the back of the capsuled paper, causing the capsules to burst under the pressure of the writing implement and to release the color former.
In one version of this process, papers coated with capsules are directly aftercoated with a developer layer (to provide two-layer self-contained paper) or capsules are directly mixed with developer and coated onto paper (to provide single-layer self-contained paper). In the case of these SC papers, an image can be created directly on the coated side by mechanical pressure, for example, by placing a sheet of uncoated paper on top and writing on it.
Carbonless copy papers are used worldwide, differing essentially only in the nature of the microcapsule wall used. Customary are gelatin, melamine-formaldehyde, and polyurethane or polyurea capsules. Polyurea capsules are customarily formed by an interfacial polyaddition process, the basic traits of which will now be outlined.
Generally, a suitable color former is dissolved in an oil by heating and a polyisocyanate is added. In addition, an aqueous solution of a protective colloid, for example, polyvinyl alcohol, is prepared. An emulsifying apparatus is then used to convert the oil and water phases into a fine oil-in-water emulsion for which the particle size is substantially equal to the average diameter of the desired capsules.
In a concurrent or subsequent operation, this emulsion is admixed with an aqueous solution of an amine or an amine-like substance (hereinafter referred to as xe2x80x9ccrosslinkerxe2x80x9d), which causes a rapid reaction of oil-dissolved isocyanate to take place at the oil/water phase boundary, to form a thin polyurea film. This polyaddition can be made virtually quantitative by supplying heat and allowing appropriate time for reaction. This process is known as an interfacial process. The end result obtained is isocyanate-free oil droplets dispersed in water which are surrounded by a resilient polyurea wall and are known as microcapsules. Typical processes of this kind and capsules resulting therefrom are described, for example, in U.S. Pat. Nos. 5,225,118, 5,164,126, EP-A780,154, U.S. Pat. Nos. 5,635,211, 5,643,506, and EP-A 535,384.
The prior art production of microcapsules having polyurea walls by an interfacial process requires a hydrophobic oil phase containing a polyisocyanate as primary wall former. The thickness of the later capsule wall is directly proportional to the fraction of the primary wall former, i.e., the polyisocyanate.
On the one hand, increasing wall thickness provides an improvement in important physical and performance properties (e.g., fracture characteristics, sensitivity to rubbing, thermal stability, etc.).
On the other hand, increasing the fraction of polyisocyanate as primary wall former entails making the oil phase and thus later micro-capsules more expensive, since suitable polyisocyanates will be high value added chemicals and hence high ticket items.
Using guanidine compounds as a crosslinker can make it possible to produce capsules having good properties, but in the case of isocyanurate-containing polyisocyanates the fraction in the oil phase and hence the wall fraction in the microcapsules is not less than 10% by weight (cf., Example 10 of EP-A 727,251).
It is an object of the present invention to provide a combination of primary wall former (polyisocyanate) and a suitable crosslinker that even in the case of less than 10% of polyisocyanate in the oil phase leads to microcapsules having properties that are equivalent to the prior art in important performance characteristics. This object is achieved by the present invention.
The present invention accordingly provides microcapsules for which the walls comprise reaction products of polyisocyanates, guanidine compounds, and optionally amines, wherein the polyisocyanates have an isocyanurate content of at least 30% by weight, based on polyisocyanate, and the walls comprise on average less than 9% by weight, based on the total weight of the microcapsule.
In a preferred embodiment, the average wall fraction comprises less than 7% by weight (especially less than 6% by weight, particularly preferably 5% by weight or less), based on the total weight of the microcapsule.